This invention relates to polycrystalline diamond and more specifically to semiconductive polycrystalline diamond that exhibits enhanced cuttability, especially Electro-Discharge Machining or Electro-Discharge Grinding cuttability.
Polycrystalline diamond (PCD) materials known in the art are typically formed from diamond grains or crystals and a ductile metal catalyst/binder, and are synthesized by high temperature/high pressure (“HTHP”) processes. Such PCD materials are ultra hard materials well known for their mechanical property of high wear resistance, making them a popular material choice for use in such industrial applications as cutting tools for machining, and subterranean mining and drilling, where the mechanical property of wear resistance is highly desired. In such applications, conventional PCD materials can be provided in the form of a surface coating, e.g., on inserts used with cutting and drilling tools, to improve wear resistance of the insert. Traditionally, PCD inserts used in such applications are produced by forming one or more layers of PCD-based material over a suitable substrate material. Such inserts, also referred to as cutting elements, comprise a substrate, a PCD surface layer, and optionally one or more transition layers to improve the bonding between the exposed PCD surface layer and the underlying substrate support layer. Substrates used in such insert applications are commonly formed from a carbide material such as tungsten carbide, WC, cemented with cobalt, Co, and commonly referred to as a cemented tungsten carbide, WC/Co system.
The layer or layers of PCD conventionally may include a metal binder therein. The metal binder is used to facilitate intercrystalline bonding between diamond grains, and acts to bond the layers to each other and to the underlying substrate. The metal binder material is generally included at a weight percentage of about 10% by weight. Metals conventionally employed as the binder are often selected from the group including cobalt, iron, or nickel and/or mixtures or alloys thereof. The binder material may also include metals such as manganese, tantalum, chromium and/or mixtures or alloys thereof. The metal binder may be provided in powder form as an ingredient for forming the PCD material, or can be drawn into the PCD material from the substrate material during HTHP process also referred to as the “sintering” process.
The amount of binder material that is used to form PCD materials represents a compromise between the desired material properties of toughness and hardness/wear resistance. While a higher metal binder content typically increases the toughness of the resulting PCD material, higher metal content also decreases the PCD material hardness, wear resistance and thermal stability. Thus, these inversely affected desired properties ultimately limit the flexibility of being able to provide PCD coatings having desired levels of both wear resistance and toughness to meet the service demands of particular applications. Additionally, when the PCD composition is chosen to increase the wear resistance of the PCD material, typically brittleness also increases, thereby reducing the toughness of the PCD material.
In many instances, after the PCD is formed, it must be cut to desired shapes for use in a cutting tool. Cutting is typically accomplished using Electro-Discharge Machining (EDM) or Electro-Discharge Grinding (EDG) operations which are well known in the art. However, because of the insulating nature of the diamond skeleton in conventional PCD it is essential to have a metallic matrix material present at the cut to ensure some conductivity of the PCD, essential to the aforementioned cutting operations. The metal binder in the PCD forms a metallic matrix and provides conductivity that supports EDM or EDG cutting. However, cooling fluid or dielectric fluid used for cooling during EDM or EDG cutting, may leach out the metal matrix from the PCD and significantly increase the resistance of the PCD layer. Various cooling/dielectric solutions such as Adcool™, and other corrosion inhibiting solutions and/or deionized water may be used during the EDM or EDG process. The electrical arcing produced between the cutting surface and the wire in EDM operations, and the grinding wheel in EDG operations, also causes leaching.
If the resistance of the PCD increases significantly due to the metal matrix in the PCD leaching out, or if areas with relatively little metal matrix are encountered, very slow or zero cutting rates may result and breakage of the cutting wire incorporated in the EDM process may occur. In some instances extra metal is provided in the PCD material to overcome this problem. Adding additional metal results in lower thermal stability of the PCD as well as reduced material hardness and a correspondingly reduced wear resistance.
Thus, a PCD material is desired that has enhanced EDM and EDG cuttability without a reduction in material hardness, wear resistance and thermal stability.